Rotating sample holder for random angle sampling in tomography

ABSTRACT

A sample holder retains a sample and can continuously rotate the sample in a single direction while the sample is exposed to a charged particle beam (CPB) or other radiation source. Typically, the CPB is strobed to produce a series of CPB images at random or arbitrary angles of rotation. The sample holder can rotate more than one complete revolution of the sample. The CPB images are used in tomographic reconstruction, and in some cases, relative rotation angles are used in the reconstruction, without input of an absolute rotation angle.

FIELD

The disclosure pertains to electron tomography.

BACKGROUND

Tomographic imaging electron microscopy is based on acquisition ofsample images at a plurality of angles of exposure. These angles aretypically set by stepping the sample through series of ascending angles.In other examples, the angles are set by tilting the sample back andforth. One difficulty with these conventional approaches is the limitedrange of tilt angles available in conventional electron microscopes. Inaddition, it can be difficult to precisely set angles, particularly inback-and-forth tilt protocols in view of the successive large angularmovements and angular direction changes. Back and forth tilting can alsobe time consuming due to the multiple starts and stops required.Furthermore, conventional ascending angular sampling schemes are notoptimal for radiation sensitive samples, where it can be beneficial tofirst sample the angles for which the sample is tilted minimally. Forthese and other reasons, alternative approaches are needed.

SUMMARY

Random angle sampling in tomographic acquisition as disclosed herein canbe beneficial for reconstruction quality and can enable reconstructionof dynamically deforming samples. In typical examples disclosed herein,stroboscopic charged particle beam (CPB) exposures of a rotating sampleholder permit random angle sampling in tomographic acquisitions. Thesample can be rotated at a constant or variable angular velocity andsample angles can be chosen by selection of exposure times. As usedherein, exposure durations are generally selected so that samplerotation during exposure does not produce unacceptable image blur, andsuch exposures are referred to herein as “stroboscopic” exposures.

Representative methods comprise rotating a sample in one direction, therotation being continuous, wherein the sample rotates through multiplerevolutions, and wherein all angles of a complete revolution of thesample are available. The sample is illuminated with a plurality ofelectron beam pulses at a pulse rate and while the sample is rotating.Alternatively, the sample can be irradiated with X-rays. In response tothe illuminating, a plurality of images of the sample is acquired, eachimage acquired with the sample at a different relative angle to at leastone of the other acquired images and the relative angle of the sample ineach acquired image is determined. In some examples, determining therelative angle of the sample is performed in concert with the respectiveacquiring of the image or after the respective acquiring of the image.In some embodiments, an absolute angle of the sample in each acquiredimage is determined based on or during a tomographic reconstruction ofthe sample. As used herein, an absolute angle is an orientation angle ofthe sample with respect to a fixed reference. In some examples,determining the relative angle of the sample in each acquired imageincludes reading an encoder coupled to a rotatable sample holder at thetime of acquisition, the rotatable sample holder rotating the sample. Asused herein, a relative angle between a first projection image acquiredat a first angle and a second projection image acquired at a secondangle is defined as a rotation needed to apply to the sample to move thesample from the first projection angle (i.e., the first projectiondirection) to the second projection angle (i.e., to the secondprojection direction. In representative examples, the determining of theabsolute angle of the sample in each acquired image is based on areading of the encoder. In other examples, the determining the relativeangle of the sample in each acquired image includes determining therelative angle based on a reconstruction of the sample or based onrotation time. The pulse rate can be variable, such as varied perrevolution or after multiple revolutions of the sample, or the pulserate can be changed after each full rotation of the sample. In someexamples, the pulse rate is increased or decreased after each fullrotation of the sample, and the rotation is at a constant or variablevelocity.

Representative apparatus comprise a rotatable sample holder operable torotate a sample continuously in one direction through multiplerevolutions, and wherein all angles of a complete revolution of thesample are available. An electron beam source is operable to irradiatethe sample with a plurality of electron beam pulses at a pulse rate andwhile the sample is rotating. A detection system operable to acquire aplurality of images of the sample corresponding to the plurality ofelectron beam pulses. The detection system can include an electrondetector situated to receive electron pulse portions responsive to theelectron beam irradiation of the sample, and wherein each image isacquired with the sample at a different relative angle to at least oneof the other acquired images. In some embodiments, a controller isconfigured to determine the relative angle or an absolute angle of thesample in each acquired image. In some examples, an encoder is coupledto the rotatable sample holder at the time of image acquisition.According to representative examples, the pulse rate of the electronbeam source is variable using random intervals, non-constant intervals,or Poisson-distributed intervals and the electron beam source can beoperable to produce electron beam pulses at a pulse rate that is changedafter each full rotation of the sample.

At least one computer readable medium containing processor-executableinstructions is configured to control an electron beam system to rotatea rotatable sample holder continuously in one direction through multiplerevolutions, and wherein all angles of a complete revolution of a samplesituated on the rotatable sample holder are available. While the sampleis rotating, the sample can be irradiated with an electron beam sourcewith a plurality of electron beam pulses at a pulse rate. A plurality ofimages of the sample is acquired corresponding to each of the pluralityof electron beam pulse, and a reconstruction of the sample based on theacquired plurality of images.

In some examples, methods include unidirectionally rotating a samplethrough an angular range that includes at least one full rotationrevolution. During the unidirectional rotation, the rotating sample isstroboscopically irradiated at a plurality of angles and sample imagesare acquired at each of the corresponding plurality of angles. In somecases, a tomographic image of the sample is produced based on the sampleimages. According to some examples, relative rotation angles or absoluterotation angles associated with the sample images are determined before,after, or during acquisition of the sample images. In some cases, therelative angles of the sample and/or the absolute angles of the sampleassociated with the sample images are determined based on a tomographicreconstruction. In further embodiments, the sample is secured to arotatable sample holder and relative rotation angles and/or absoluterotation angles are determined with an encoder coupled to the rotatablesample holder. In some examples, the rotating sample is stroboscopicallyirradiated at a fixed pulse rate or at a variable pulse rate to producepulses at fixed or variable time intervals. Variable pulse intervals canbe produced with stepped or chirped pulse rates. Alternatively, randomtime intervals can be used, for example, time intervals specified by anexponential distribution or other distribution. As used herein, “pulserate” refers to a number of pulses per unit time and can be fixed,stepped, chirped or otherwise a deterministic or probabilistic functionof time. For example, a pulse rate can be chirped at a fixed or variablerate, or a pulse chirp rate can be randomly selected. In some examplespulses are applied at intervals that are random as specified with, forexample, a probability distribution such as the Poisson distribution, anexponential distribution, or other distribution. Some of thesedistributions are characterized with parameters that are referred to asrate parameters, but these rate parameters do not necessarily correspondto actual pulse rates (pulses/time) although such actual pulse rates andaverage pulse rates can be determined with a distribution. Rateparameters or other parameters that specify distributions can be fixed,but can be variable as well. For example, for pulse rates or pulseintervals (x) specified by an exponential distribution ƒ(x; λ)=λe^(−λx),wherein λ is rate parameter, this rate parameter can be a function oftime as well, i.e., λ=λ(t). In further examples, the stroboscopicexposures of the rotating sample are at a variable pulse rate that isdetermined based on an angle or number of rotations of the rotatingsample. According to representative examples, the sample isunidirectionally rotated through an angular range that includes aplurality of full rotations.

Representative apparatus include a switchable charged particle beam(CPB) source and a rotatable sample stage operable to rotate a sample byat least one complete revolution. A controller is coupled to the CPBsource and the rotatable sample stage and configured to expose a samplesituated on the rotatable sample stage at a plurality of angles during aunidirectional rotation of the sample, wherein the plurality of exposureangles are in a range greater than 360°. In some examples, thecontroller is coupled to activate the CPB source at the plurality ofangles during the unidirectional rotation of the sample tostroboscopically expose the sample to the CPB. In other examples,controller is coupled to blank the CPB so that the sample isstroboscopically exposed to the CPB at the plurality of angles duringthe unidirectional rotation of the sample. According to someembodiments, a rotational encoder is coupled to the rotatable samplestage and provides at least one of a relative rotation angle and anabsolute rotation angle for each of the exposure angles. In someexamples, the controller establishes the stroboscopic exposures at afixed or variable pulse rate or a random times. In further examples, atleast one detector is coupled to detect radiation produced in responseto CPB exposures of the sample and produce corresponding images. Forenergy dispersive X-ray like applications, the at least one detector isan X-ray detector while in other examples, the detector is a CPBdetector. In embodiments, the controller is coupled to produce atomographic reconstruction corresponding to the sample based on thetomographic images and the plurality of angles and produce estimates ofthe angles of the plurality of angles during tomographic reconstruction.

Electron beam apparatus comprise a controller situated to be coupled toan electron beam source. At least one computer-readable medium iscoupled to the controller and contains controller-executableinstructions to cause the controller to direct the electron beam sourceto stroboscopically irradiate a sample at a plurality of arbitraryangles during a unidirectional rotation of the sample, and acquire asample image at each of the corresponding plurality of angles. Accordingto some examples, the at least one computer-readable medium furthercontains controller-executable instructions to reconstruct a tomographicimage of the sample based on the sample images and to determine relativerotation angles and/or absolute rotation angles associated with thesample images. In some examples, the relative rotation angles and/or theabsolute rotation angles associated with the sample images aredetermined after acquisition of the sample images or are determinedbased on a tomographic reconstruction. In additional examples, the atleast one computer-readable medium further containscontroller-executable instructions for determining relative rotationangles and/or absolute rotation angles with an encoder coupled to arotatable sample holder. In other representative embodiments, thecontroller is coupled to direct the electron beam source tostroboscopically irradiate the sample at a plurality of arbitrary anglesduring the unidirectional rotation of the sample or at a fixed orvariable pulse rate based on random time intervals such as Poissondistributed intervals, wherein an angular range of the arbitrary anglesincludes a plurality of full rotations.

The foregoing and other features of the disclosed technology will becomemore apparent from the following detailed description, which proceedswith reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a representative CPB microscope that includes arotatable sample stage.

FIGS. 1B-1D illustrate representative sample rotations obtained with theCPB microscope of FIG. 1A.

FIG. 1E illustrates a representative needle or pillar shaped samplesituated for rotation.

FIG. 2 illustrates another representative system for acquiring randomangle CPB images for tomography with a continuously rotatable samplestage.

FIG. 3 illustrates a representative system for acquiring random angleCPB images for tomography with a continuously rotatable sample stage.

FIG. 4 illustrates a representative method of producing tomographicreconstructions using images acquired with random angle stroboscopicexposures of a rotating sample.

FIG. 5 illustrates a representative method of acquiring sample imagesbased on random exposure times as a function of sample rotation speed.

FIG. 6 illustrates a representative system for acquiring CPB images witha sample stage that is rotatable with a DC motor.

FIG. 7 illustrates another method of acquiring CPB images fortomographic reconstruction.

FIG. 8 illustrates a representative computing environment foracquisition and analysis of images for tomographic reconstruction.

DETAILED DESCRIPTION

Disclosed herein are methods and apparatus for charged particletomography. Typically, a sample is situated on a rotatable sample stagefor repetitive exposure to a charged particle beam (CPB). The disclosedexamples are generally described with reference to transmission electronmicroscopy, but other CPBs can be used. Alternatively, samples can beirradiated with X-rays and X-ray based images acquired. In someexamples, stroboscopic illumination of a rotating sample at random orother sequences of angles is used to acquire a sequence of images to beused in tomographic reconstruction. A sample can be rotated at a uniformangular velocity, and random angular exposures can be established basedon one or more series of angular values which can be generated as neededor retrieved from a computer readable storage such as memory. Angles canbe specified based on a phase associated with rotation of a sample, aset of exposure times based on a sample rotation speed, generatedrandomly during image acquisition, or otherwise specified. The samplecan be rotated at a fixed or variable velocity and stroboscopicallyirradiated while rotating. The stroboscopic irradiation can be at afixed or variable pulse rate including at random times durationrotation. The irradiation angles can be determined prior to, during, orafter irradiation and the irradiation angles can be a fixed or variabledistribution of angles, including random angles.

As used herein, “column” refers generally to one or more CPB opticalelements or combinations of elements such as CPB sources, CPB lenses,CPB deflectors, CPB apertures, stigmators, or other CPB opticalelements. One of such optical elements can be used to produce a pulsedCPB that can be directed to a sample to provide a pulsed exposure. Suchpulsed exposures are generally referred to as “stroboscopic” exposuresto indicate that effective exposure times are sufficiently short withrespect to sample rotation that suitable images are produced, i.e.,without undue motion-induced blurring. Suitable exposure times cancorrespond to rotations of less than 0.0001, 0.001, 0.01 degrees orother angles. Specification of any permitted maximum exposure durationcan depend on image magnification and intended resolution. In thedisclosed examples, a CPB or a CPB column is energized to produce astroboscopic CPB exposure, but a continuous CPB can be used withstroboscopic detection, i.e., detection of charged particles orelectromagnetic radiation produced in response to CPB exposure andreceived in a detection time window, referred to herein as“stroboscopic” detection. In stroboscopic exposures, a CPB may have acontinuous component in addition to the stroboscopic component. In manypractical examples, pulsed exposures are preferred in order to reducesample degradation produced by CPB exposures that include a continuouscomponent. A continuous component can contribute to undesirable samplechanges without improving tomographic imagery.

In some examples, sample images are acquired using exposures at aplurality of angles such as random angles or random angular differencesor deterministic angles or deterministic angular differences. As usedherein, random or a random selection refers to values that are unevenlyspaced and can be selected deterministically or using a random or pseudorandom number generator or otherwise approximate randomly selectedvalues. It will be appreciated that any set of such random values cangenerally be selected with a so-called pseudorandom number generator.One or more sets of random values can be used and different sets caninclude different values and/or different numbers of values. Valuesassociated with angles, exposure times, or phases can be determinedbased on a corresponding set of random numbers and the random numbers ofthe set processed to establish corresponding angles, exposure times, orphases. For example, if a set of N random numbers R_(i) between 0 and 1is obtained, angles α_(i) can be selected as πR_(i) radians, 2πR_(i)radians, or, more generally, AπR_(i) radians wherein Aπ radians is atotal angular range to be used. The angles α_(i) can be specified aspositive and negative, and exposure at any particular angle α_(i) caninclude multiple rotations, i.e., α_(i) is a rotation angle in radiansmodulo-2π or modulo-π. Exposure times, relative times, phases, andrelative phases can be similarly specified based on a set of randomnumbers. Values can also be generated on the fly as needed using arandom number generator. In some examples, the selected or generatedvalues are used in image reconstruction and each image of a set isassociated with a respective random number with particular value.

In some examples, sample exposures are made using a constant samplerotation speed to produce uniformly spaced exposures for convenience,but non-uniform speeds such as monotonically increasing or decreasingspeeds, or arbitrary increasing and decreasing rotation speeds can beused. With a uniform, constant rotation, samples can be acquired atrandom exposure angles with suitable pulse rates or pulse intervals. Asdiscussed above, such random exposures can be based on random exposuretimes or rotation phases that can be stored or generated as needed.Alternatively, sample rotation can be at a variable speed such as arandom speed, and exposure times can be separated by a constant delay.

In some examples, a rotation speed is constant or variable and thestroboscopic exposures can have different pulse distributions, such asrandom, at fixed or variable pulse rates, or combinations thereof. Pulseintervals can be random, fixed, variable, or combinations thereof sothat angular intervals can similarly be constant, variable, random orcombinations thereof. Relative and/or absolute exposure angles can bedetermined after acquisition of some or all images.

EXAMPLE 1

Referring to FIG. 1A, a CPB system 100 includes a CPB emitter 102 thatcan include a field emitter 104 or other emission source that produces aCPB from an emitter tip 106. CPB current can be controlled by one ormore of a voltage applied to the field emitter 104 or emitter tip 106 asprovided by an emitter drive 107. A suppressor electrode 108 is situatedabout the field emitter 104, typically to suppress stray chargedparticle emissions, and an extractor electrode 110 is situated toestablish a voltage with respect to the emitter tip 106 to induce aselected CPB current. A beam current drive 112 is coupled to thesuppressor electrode 108 and the extractor electrode 110. As shown inFIG. 1A, any or all of the suppressor electrode 108, the extractorelectrode 110, and the field emitter 104 or emitter tip 106 can becontrolled with the emitter drive 107 or the beam current drive 112 toproduce a pulsed or other variable CPB so that a sample S can bestroboscopically exposed. One or more additional beam apertures can besituated along a CPB system axis 119 and can be used to control the CPBby, for example, blocking the CPB except at predetermined exposuretimes. For example, an aperture defined in an aperture plate 118 can beused to block or attenuate a CPB in response to application of adeflection voltage from a deflection driver 120 to a beam deflector 122,such as a resonant beam deflector. With the beam deflector 122activated, a CPB 126 is deflected to be blocked by the aperture plate118. Additional apertures and deflectors can be provided, but are notshown in FIG. 1A. The example of FIG. 1A can also provide beammodulation using a gun lens, but such a lens is not shown. In additionto the CPB modulations provided by driving one or more CPB lenses,deflectors, aperture plates or other CPB optical elements, pulsed CPBemissions can be produced in response to irradiation of a suitabletarget with a pulsed or modulated optical beam or beams. Such CPBs canbe further modulated using CPB optical elements as needed.

The sample S is situated on a sample stage 130 that is rotatable aboutan axis 134 to an desired angle α in response to activation of a motoror other mechanism 132. In some embodiments, all angles of a sample maybe available due to the continuous rotation of the sample. While allangles are available, some angles, depending on sample shape, may beavoided due to limited data collection. For example, angles that presenta face of a sample that requires the CPB to transmit through arelatively long portion of the sample may be undesirable. Typically, thesample is adjusted through a plurality of angles and correspondingstroboscopic (pulsed) electron beam emissions applied to produce chargedparticles (e.g., scattered CPB portions, secondary electrons) orelectromagnetic radiation (e.g., X-rays) that are received by a detector140 to produce corresponding images. These images can be subsequentlytomographically processed. A controller 142 is coupled to producestroboscopic CPB emissions and position the sample S at a plurality ofangles with respect to the axis 119. The angles can be random angles andcan include one or more complete rotations of the sample S about theaxis 119. The controller generally controls pulse timing and pulsedistributions to obtain images at desired angles.

FIGS. 1B-1D illustrate unidirectional rotations of the sample S toprovide arbitrary rotations. In these examples, an initial near-normalincidence exposure can be used so that any sample degradation for thisexposure can be reduced. For convenience, effective rotation angles αare deemed positive for clockwise rotations of the sample S that areless than π/2; effective rotation angles α are deemed negative forcounter-clockwise rotations of the sample S that are less than π/2. Forconvenience, effective rotation angles are reference to the CPB systemaxis 119. Arbitrary sample rotations starting at an arbitrary initialrotation angle can be achieved using continuous clockwise orcounterclockwise rotations. As shown in FIG. 1B, the sample S issituated so that the CPB system axis 119 and a sample surface normal 129are substantially parallel, i.e., α₁=0. In FIG. 1C, the sample S isrotated clockwise so that the sample S is situated at an angle α₂>0 withrespect to the CPB system axis 119. In FIG. 1D, the sample S is rotatedfurther clockwise by an angle 2π-|α₃| so that the sample S is situatedat an angle α₃<0 with respect to the CPB system axis 119. Additionalrotation angles can be obtained using additional complete rotationrevolutions of the sample S, without changing a direction of rotation. Aclockwise or counterclockwise rotation can be used to obtain anarbitrary set of sample rotation angles α₁, . . . , α_(N) and anyparticular angle can be used for an initial exposure. These rotationangles can all be provided with a unidirectional rotation, as desired.

The sample S in FIGS. 1A-1D is shown as a lamella, but other shapes canbe used. As shown in FIG. 1E, a sample S′ can have a column, pillar, orneedle shape and similarly rotated

In some examples, images associated with smaller tilt angles areacquired prior to acquisition of images at larger tilts. For example,exposures at a series of small tilts (both clockwise andcounterclockwise with respect to a CPB system axis) can be obtainedinitially, and some exposures may require unidirectional samplerotations of more than 360 degrees or multiple revolutions. Tilt anglesα are generally obtained modulo 360 degrees. Exposures at larger tiltscan then be made and such exposures may require unidirectional samplerotations of more than 360 degrees.

EXAMPLE 2

Referring to FIG. 2, a CPB microscope or other CPB imaging system 200includes a sample stage 202 that is situated to rotate a sample S aboutan axis 201. In typical examples, such rotation is continuous androtation angles can be as great as 180 degrees, 360 degrees, 720 degreesor other values. A stage driver 206 is actuatable to produce suchrotations of the sample stage 202. The sample stage 202 and/or the stagedriver 206 are coupled to a rotary encoder 208 that permitsdetermination of specimen rotation angles. The stage driver 206 and therotary encoder 208 are generally coupled to a controller 210 that caninitiate or regulate stage rotation. The controller 210 is also coupledto a memory 212 that stores a sequence of values that define respectivestroboscopic exposures. Such values can be stored as, for example, aseries of exposure times, sample tilt angles, or relative phases or timedifferences between exposures. In some examples, actual exposure timesare calculated based on predetermined rotations of the sample or basedon a fixed or variable rotation speed of the sample stage 202.

The controller 210 is coupled to a CPB source and/or column 213 toproduce a pulsed CPB 214 based on the stored sequence of values. Adetector 218 is situated to receive charged particles or electromagneticradiation produced in response to a the pulsed CPB 214 such as scatteredelectrons, secondary electrons, X-rays, or other charged or neutralparticles, or other electromagnetic radiation. The detector 218 iscoupled to the controller 210 so that images associated with the pulsedexposures can be stored for tomographic processing usingcomputer-executable instructions stored in a memory 220. Alternatively,the controller 210 can communicate the images via a wired or wirelessnetwork to an arbitrary location for tomographic processing,reconstruction, and display.

EXAMPLE 3

Referring to FIG. 3, a CPB imaging system 300 includes a sample stage302 that is configured to receive a sample S. A stage driver 304 iscoupled to the sample stage 302 to produce a sample rotation about anaxis 301. In most cases, translational motion of the sample S with thesample stage 302 is provided, but translational components are not shownin detail in FIG. 3. The stage driver 304 can produce a sample rotationat a fixed frequency ƒ. This frequency can be set with one or more useror controller inputs to the stage driver 304, but can also be fixedinternally. In most cases, initiation of sample rotation is provided bya user input in preparation for image acquisition. As shown, the stagedriver 304 can provide an indication of the fixed frequency fat one ormore outputs. A pulse sequencer 310 is coupled to a CPB source or column312 to produce stroboscopic exposure of the sample S at a sequence ofsample angles or angular differences. The pulse sequencer 310 can alsooutput data values of other indications of times or relative times ofstroboscopic exposures. A detector 314 receives radiation responsive tothe stroboscopic exposures and produces corresponding sample images thatcan be output for remote or local processing to produce tomographicimages. As shown in FIG. 3, sample angles are not measured andreconstruction is based on the sequence of angles or angulardifferences.

EXAMPLE 4

Referring to FIG. 4, a representative method 400 includes selecting apulse distribution at 402 which can be associated with sample angles α₁,. . . , α_(N), wherein N is a positive integer. The pulse distributioncan be specified directly as exposure times, exposure rates orcombinations of pulse rates, or random times. Exposure times can alsodirectly associated with angles, as relative phases with respect tosample rotation, or otherwise specified as may be convenient. The anglesare typically random. At 404, stroboscopic images are acquired for eachof the pulses of the pulse distribution with a unidirectional samplerotation. In some cases, the pulse distribution produces sample anglesα₁, . . . , α_(N) that are specified (in radians) modulo-2π, modulo-π orotherwise, and can be achieved with one or more rotations greater than90 degrees, 180 degrees, or 360 degrees The stroboscopic images for eachof the pulses are then processed to produce a tomographic reconstructionat 406.

EXAMPLE 5

Referring to FIG. 5, a method 500 includes establishing a samplerotation speed ƒ(t) at 502, typically a constant speed, i.e.,ƒ(t)=constant, but arbitrary, variable rotation speeds can be used.Generally, unipolar speeds are preferred (ƒ(t)>0 radian/sec) so thatsample rotation is in a single direction during exposures.Unidirectional motion reduces or eliminates rotational artifactsassociated with direction changes, and can permit more rapid samplerotations and thus imaging of the sample at successively more rapidlyvarying (relative) angles. At 504, exposure times/pulse distributionsare selected. The exposure times can be established using random numbersthat are then scaled to produce the random exposure times. For either afixed or variable ƒ(t), sample exposure times can be uniformly spaced orbe based on a combination of uniform and random spacing, or some otherdistribution. Random rotations can be used as well, but are not shown inFIG. 5 as it is generally more convenient to maintain a more or lessconstant rotation speed during exposure. At 506, stroboscopic exposuresare used to obtain images for each of the exposure times, and the imagescan be communicated for tomographic processing or storage at 508. Insome cases, the associated exposure angles are not communicated alongwith the images, but typically each image and an associated angle arecommunicated.

EXAMPLE 6

With reference to FIG. 6, a CPB imaging system 600 includes a samplestage 602 that is configured to retain a sample S. The sample stage 602is coupled to a DC motor 604 so that the sample S is rotatable throughan arbitrary angle α about an axis 606 that is non-parallel (typicallyperpendicular) to a CPB exposure axis 608. An encoder 610 provides anindication of rotation angle to a controller 612 that is coupled to aCPB source or CPB column 614 that provides stroboscopic exposures of thesample S to a pulsed CPB 615. A detector 616 is situated to receiveelectromagnetic radiation or charged particles 620 from the sample S inresponse to exposure to the pulsed CPB 615. For example, X-rays,secondary electrons, scattered electrons or other scattered, reflected,diffracted charged particles can be produced. The detector 616 iscoupled to the controller 612 and provides image data to the controller612. The controller 612 can be configured to process the receivedimages, direct the received images for remote processing, as well ascontrolling the CPB source/column 614, the DC motor 604, the samplestage 602 as well as receiving rotation data from the encoder 610.

EXAMPLE 7

Referring to FIG. 7, a method 700 of acquiring images for tomographicprocessing includes establishing initial rotation angles and a rotationfrequency at 702. At 704, a counter I is initialized, wherein I is apositive integer, and at 706, a sample is exposed at a time T_(I). At708, an image obtained in response to the exposure is stored, and at 710it is determined if additional exposures are to be obtained. If so, at712, the counter I is incremented and exposure and image storage arerepeated. Upon completion of exposures, at 714, the acquired images arecommunicated for tomographic reconstruction.

EXAMPLE 8

FIG. 8 and the following discussion are intended to provide a brief,general description of an exemplary computing environment in which thedisclosed technology may be implemented. Although not required, thedisclosed technology is described in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by a personal computer (PC). Generally, program modules includeroutines, programs, objects, components, data structures, etc., thatperform particular tasks or implement particular abstract data types.Moreover, the disclosed technology may be implemented with othercomputer system configurations, including hand-held devices,multiprocessor systems, microprocessor-based or programmable consumerelectronics, network PCs, minicomputers, mainframe computers, and thelike. The disclosed technology may also be practiced in distributedcomputing environments where tasks are performed by remote processingdevices that are linked through a communications network. In adistributed computing environment, program modules may be located inboth local and remote memory storage devices.

With reference to FIG. 8, an exemplary system for implementing thedisclosed technology includes a general purpose computing device in theform of an exemplary conventional PC 800, including one or moreprocessing units 802, a system memory 804, and a system bus 806 thatcouples various system components including the system memory 804 to theone or more processing units 802. The system bus 806 may be any ofseveral types of bus structures including a memory bus or memorycontroller, a peripheral bus, and a local bus using any of a variety ofbus architectures. The exemplary system memory 804 includes read onlymemory (ROM) 808 and random access memory (RAM) 810. A basicinput/output system (BIOS) 812, containing the basic routines that helpwith the transfer of information between elements within the PC 800, isstored in ROM 808.

The exemplary PC 800 further includes one or more storage devices 830such as a hard disk drive for reading from and writing to a hard disk, amagnetic disk drive for reading from or writing to a removable magneticdisk, and an optical disk drive for reading from or writing to aremovable optical disk (such as a CD-ROM or other optical media). Suchstorage devices can be connected to the system bus 806 by a hard diskdrive interface, a magnetic disk drive interface, and an optical driveinterface, respectively. The drives and their associatedcomputer-readable media provide nonvolatile storage of computer-readableinstructions, data structures, program modules, and other data for thePC 800. Other types of computer-readable media which can store data thatis accessible by a PC, such as magnetic cassettes, flash memory cards,digital video disks, CDs, DVDs, RAMs, ROMs, and the like, may also beused in the exemplary operating environment.

A number of program modules may be stored in the storage devices 830including an operating system, one or more application programs, otherprogram modules, and program data. A user may enter commands andinformation into the PC 800 through one or more input devices 840 suchas a keyboard and a pointing device such as a mouse. Other input devicesmay include a digital camera, microphone, joystick, game pad, satellitedish, scanner, or the like. These and other input devices are oftenconnected to the one or more processing units 802 through a serial portinterface that is coupled to the system bus 806, but may be connected byother interfaces such as a parallel port, game port, or universal serialbus (USB). A monitor 846 or other type of display device is alsoconnected to the system bus 806 via an interface, such as a videoadapter. Other peripheral output devices, such as speakers and printers(not shown), may be included.

The PC 800 may operate in a networked environment using logicalconnections to one or more remote computers, such as a remote computer860. In some examples, one or more network or communication connections850 are included. The remote computer 860 may be another PC, a server, arouter, a network PC, or a peer device or other common network node, andtypically includes many or all of the elements described above relativeto the PC 800, although only a memory storage device 862 has beenillustrated in FIG. 8. The personal computer 800 and/or the remotecomputer 860 can be connected to a logical a local area network (LAN)and a wide area network (WAN). Such networking environments arecommonplace in offices, enterprise-wide computer networks, intranets,and the Internet.

When used in a LAN networking environment, the PC 800 is connected tothe LAN through a network interface. When used in a WAN networkingenvironment, the PC 800 typically includes a modem or other means forestablishing communications over the WAN, such as the Internet. In anetworked environment, program modules depicted relative to the personalcomputer 800, or portions thereof, may be stored in the remote memorystorage device or other locations on the LAN or WAN. The networkconnections shown are exemplary, and other means of establishing acommunications link between the computers may be used.

As shown in FIG. 8, the memory 810 includes portions 804A, 806B, 806Cthat store computer-executable instructions for generating randomsequences (or storing one or more such sequences), storing image data,and stage and CPB system control, respectively. Communication with a CBPsystem or components associated with a CPB system is provided with oneor more analog-to-digital convertors (ADCs) 870 or one or moredigital-to-analog convertors (DACs) 871.

General Considerations

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

In some examples, values, procedures, or apparatus are referred to as“lowest”, “best”, “minimum,” or the like. It will be appreciated thatsuch descriptions are intended to indicate that a selection among manyused functional alternatives can be made, and such selections need notbe better, smaller, or otherwise preferable to other selections.

Examples are described with reference to directions indicated as“above,” “below,” “upper,” “lower,” and the like. These terms are usedfor convenient description, but do not imply any particular spatialorientation.

The term “image” is used herein to refer to displayed image such as on acomputer monitor, or digital or analog representations that can be usedto produce displayed images. Digital representations can be stored in avariety of formats such as JPEG, TIFF, or other formats. Image signalscan be produced using an array detector or a single element detectoralong with suitable scanning of a sample. In most practical examples,images are 2 dimensional.

Sample stage rotations can be provided with motors and actuators ofvarious kind, including DC motors, stepper motors, rotary piezoelectricmotors, AC motors, or other devices. Rotation angles can be detectedwith optical encoders, magnetic encoders or other devices. Continuousrotation refers to rotations that are allowed to proceed during imageacquisition. For example, sequences of drive signals applied to astepper motor to produce a rotation can continue during imageacquisition and a waiting time at a selected imaging angle is notneeded. Although a stepper motor is used, the rotation is referred to ascontinuous. In other examples, a continuous (i.e., non-stepper) motor isallowed to freely rotate the sample. Images can be acquired at randomtimes. In some examples random time intervals are selected based on aPoisson distribution. In this case, a smallest time interval Δt and amean time interval NΔt are selected and the associated Poissondistribution is defined as:

${{P(n)} = {\frac{{\overset{\_}{N}}^{n}}{n!}e^{- \overset{¯}{N}}}},$wherein n is an integer number of time intervals and sample timeintervals for image acquisition are selected as nΔt. Such time intervalscan be implemented using a suitable random process defined usingcomputer-executable instructions such as random or pseudorandom numbers.Random time intervals Δt can also be based on an exponentialdistribution, wherein P(Δt)=λe^(−λΔt), wherein λ⁻¹ is a mean pulseinterval. It should be noted that values that characterize somedistributions, such as the value λ for the exponential distribution aretypically referred to as “rate parameters” or “rates” but such values donot correspond to actual pulse rates (e.g., pulses/sec). Similarly, thePoisson distribution is generally characterized with a parameter λ thatis an average number of events in a particular interval. This rate alsodoes not correspond to an actual pulse rate.

In some cases, an angular range is coarsely sample and then refined. Forexample, angular steps of 20° are used followed by steps of 5° (skippingpreviously sampled angles), and then followed by steps of 1° (skippingpreviously sampled angles). In another example, referred to as “dosesymmetric,” acquisition starts at a sample tilt of 0°, followed byangles of +°2, −2°, +4°, −4°, +6°, −6° etc. In still other examples, anangle or angles can be chosen using on-the-fly reconstruction which candetect which angle would contain most useful information.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples and should not betaken as limiting. We therefore claim all that comes within the scopeand spirit of the appended claims.

We claim:
 1. A method comprising: rotating a sample in one direction,the rotation being continuous, wherein the sample rotates throughmultiple revolutions, and wherein all angles of a complete revolution ofthe sample are available; illuminating the sample with a plurality ofelectron beam pulses, the illuminating performed at a pulse rate andwhile the sample is rotating; in response to the illuminating, acquiringa plurality of images of the sample, each image acquired with the sampleat a different relative angle to at least one of the other acquiredimages; and determining the relative angle of the sample in eachacquired image.
 2. The method of claim 1, wherein determining therelative angle of the sample is performed in concert with the respectiveacquiring of the image.
 3. The method of claim 1, wherein determiningthe relative angle of the sample is performed after the respectiveacquiring of the image.
 4. The method of claim 1, further includingdetermining an absolute angle of the sample in each acquired image basedon or during a tomographic reconstruction of the sample.
 5. The methodof claim 1, wherein determining the relative angle of the sample in eachacquired image includes reading an encoder coupled to a rotatable sampleholder at the time of acquisition, the rotatable sample holder rotatingthe sample.
 6. The method of claim 5, further includes determining anabsolute angle of the sample in each acquired image based on a readingof the encoder.
 7. The method of claim 1, wherein determining therelative angle of the sample in each acquired image includes determiningthe relative angle based on a reconstruction of the sample.
 8. Themethod of claim 1, wherein determining the relative angle of the samplein each acquired image includes determining the relative angle based onrotation time.
 9. The method of claim 1, wherein the pulse rate isvariable.
 10. The method of claim 9, wherein the pulse rate is variedper revolution or after multiple revolutions of the sample.
 11. Themethod of claim 1, wherein illuminating the sample with a plurality ofelectron beam pulses includes changing the pulse rate after each fullrotation of the sample.
 12. The method of claim 11, wherein changing thepulse rate after each full rotation of the sample includes increasingthe pulse rate.
 13. The method of claim 11, wherein changing the pulserate after each full rotation of the sample includes decreasing thepulse rate.
 14. The method of claim 1, wherein the rotation is at aconstant or variable velocity.
 15. The method of claim 1, wherein thepulse rate is constant.
 16. An apparatus, comprising: a rotatable sampleholder operable to rotate a sample continuously in one direction throughmultiple revolutions, and wherein all angles of a complete revolution ofthe sample are available; an electron beam source operable to irradiatethe sample with a plurality of electron beam pulses at a fixed,variable, or random pulse rate and while the sample is rotating; adetection system operable to acquire a plurality of images of the samplecorresponding to the plurality of electron beam pulses; and a controllerconfigured to determine the relative angle or an absolute angle of thesample in each acquired image.
 17. The apparatus of claim 16, whereinthe detection system includes an electron detector situated to receiveelectron pulse portions responsive to the electron beam irradiation ofthe sample, and wherein each image is acquired with the sample at adifferent relative angle to at least one of the other acquired images.18. The apparatus of claim 16, further comprising an encoder coupled tothe rotatable sample holder at the time of image acquisition.
 19. Themethod of claim 1, further comprising varying the pulse rate usingrandom intervals or non-constant intervals.
 20. The method of claim 1,further comprising changing the pulse rate after each full rotation ofthe sample.
 21. At least one computer readable medium containingprocessor-executable instructions configured to control an electron beamsystem to: rotate a rotatable sample holder sample continuously in onedirection through multiple revolutions, and wherein all angles of acomplete revolution of a sample situated on the rotatable sample holderare available; while the sample is rotating, irradiate the sample withan electron beam source with a plurality of electron beam pulses at apulse rate; acquire a plurality of images of the sample corresponding toeach of the plurality of electron beam pulses; and produce areconstruction of the sample based on the acquired plurality of images.